The atmosphere surrounding the Earth contains something called "greenhouse gases." Certain gases that exist naturally in the atmosphere trap heat that would otherwise escape into space. It's like building a blanket over the Earth. And it's a good thing, too, because otherwise the Earth would be 60 degrees colder than it is now.
The problem is human beings are thickening that blanket by adding air pollution to the atmosphere. It's estimated that burning oil and other fossil fuels have pumped over 700 billion tons of carbon dioxide pollution into the atmosphere! And that's what experts reason is causing the Earth to warm—a thicker blanket means a warmer planet.